Electrochemical cell for hybrid electric vehicle applications

ABSTRACT

Embodiments of the present invention are directed an electrochemical energy storage device, such as a cell or a battery, that includes segmented stackable bus bars for stacking electrodes, the bus bar segments extending a substantial length of an edge of the electrodes to provide proper inter-electrode spacing, substantially uniform electrochemical potential and current density between electrodes, efficient internal heat dissipation and desired electrode structural rigidity, and, optionally, a compression member, separate from the case, to compress the stacked electrodes.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefits, under 35 U.S.C.§119(e), ofU.S. Provisional Application Ser. No. 60/712,762, filed Aug. 29, 2005,entitled “Electrochemical Cell for Hybrid Electric Vehicle Applications”to Swan and which is incorporated herein by this reference.

FIELD

The present invention relates generally to a method of constructingelectrochemical energy storage devices for improved lifetime underpartial state-of-charge operation, enhanced dissipation of internallygenerated heat and improved stability to shock and vibration loads.

BACKGROUND

Large energy storage battery systems are known, for example, from dieselsubmarines. In this application, a pack of large energy storagebatteries are used to provide all-electric power. These are designed toprovide high energy storage capacity for extended underwater operationsduring which the battery pack cannot be recharged. Battery pack cost andlifetime are generally not major concerns.

Large energy storage battery systems have also been used as standbypower sources and for power regulation in a number of applications. Asan example, a large stationary battery system was installed at theisland village of Metlakatla, Alaska in the late 1990s. The 1.4megawatt-hour, 756 volt battery system was designed to stabilize theisland's power grid providing instantaneous power into the grid whendemand was high and absorbing excess power from the grid to allow itshydroelectric generating units to operate under steady-state conditions.Because the battery pack is required to randomly accept power as well asto deliver power on demand to the utility grid, it is continuouslyoperated at between 70 and 90% state-of-charge. Equalization charges areconducted during maintenance periods scheduled only twice each year.

It has been possible to assess aging and performance capabilities overtime in this controlled cycling type of service by detailed monitoring.Data has been generated to demonstrate the long-term viability of cellsin this type of use, performing functions such as load leveling, peakshaving and power quality enhancement. Detailed examination of the cellsplates and separators have shown little wear indicating that controlledoperation such as described above can result in battery lifetimes thatcan approach design lifetimes associated with float service.

Large capacity (over about 400 A-hrs) lead-acid cells, for example, aretypically designed for standby use applications characterized by:

-   1. maintaining close to a full state of charge (float charge    condition);-   2. low discharge rates (typically about C/20);-   3. lifetime limited by calendar life where the cell life is    terminated by internal corrosion, water loss;-   4. lifetime not limited by ampere-hour throughput; and-   5. short cell string length (24 to 36 cells electrically connected    in series).

It has long been thought that to achieve optimum life and performancefrom a lead-acid battery, it is necessary to float the battery underrigid voltage conditions to overcome self-discharge reactions whileminimizing overcharge and corrosion of the cell's positive grid. Thishas resulted in batteries being used primarily in a standby mode.

The use of energy storage batteries in combination with a generator isknown for automobiles, buses and other road and highway vehicles.Electric batteries have been used to store electric power to driveelectric locomotives as, for example, disclosed by Manns in U.S. Pat.No. 1,377,087 which is incorporated herein by reference. Donnelly hasdisclosed the use of a battery-dominant hybrid locomotive in U.S. Pat.No. 6,308,639 which is also incorporated herein by reference.

One of the principal objectives of hybrid locomotive design is tooperate the locomotive in such a way as to maximize the lifetime of itsenergy storage unit. This is because the cost structure of an energystorage unit such as for example a battery pack or capacitor bank isprimarily one of capital cost and secondarily of operating costs. It isknown, for example, that operating a lead-acid battery pack in apreferred state-of-charge (“SOC”) range or with a preferred chargingalgorithm or with both tends to extend serviceable lifetime of cells incyclical service towards that of float service. However, this mode ofoperation limits the effectiveness of a hybrid vehicle such as alocomotive that has high power demands, requires large storage capacityand often requires large reductions in state-of-charge at intermediateor high power.

A hybrid electric vehicle (“HEV”) application is typically characterizedby:

-   1. maintaining a variable partial state of charge during operation;-   2. high discharge rates from C/5 to 2 C;-   3. lifetime limited by amp-hour throughput;-   4. lifetime not limited by calendar life; and-   5. very long cell strings (several hundred cells electrically    connected in series).

Operation of large series strings of electrically series-connectedlead-acid batteries under hybrid locomotive operating conditions hasresulted in substantially shorter cell lifetimes due to prematurecapacity loss. Premature capacity loss can result from, for example:

-   -   high resistance at the interface of the active material and grid        surface of the positive plate;    -   expansion and contraction of body of active material on the        positive plate causing a progressive loss of cohesion at the        interface of the active material and grid surface;    -   sulfation on the negative plate.

When any of these conditions lead to premature capacity loss, itgenerally signals the end of the useful lifetime of a cell or cells in aseries string. Even the onset of any of these conditions can upset thebalance of other cells in a string and accelerate premature capacityloss in the entire pack as a result of thermal and chemical imbalances.In a hybrid locomotive, for example, cells are subjected to shock andvibration loadings and are operated in widely varying ambient thermalenvironments. These can lead to cell failure because of, for example,shorting due to active particles being dislodged and moving around;ground faults from acid mist venting and/or case cracking; and largetemperature variation amongst the cells in the pack.

Another problem with, for example, large energy storage lead-acid cellsis stratification of the electrolyte when the cells are oriented withtheir plates in a vertical position. This is often a problem withsulphuric acid electrolytes in a separator matrix at high charging ordischarging rates causing local hot spots to develop and change theconcentration of the electrolyte. This problem can be overcome by usinga gel electrolyte. The disadvantage of gel electrolyte cells is that thegel electrolyte tends to have a high internal resistance and so maylimit the power output of a large energy storage cell, especially in aHEV application. Alkaline cells typically do not have an electrolytestratification problem because the alkaline salt in the solution doesnot participate in the cell reaction.

High capacity, high power cells for use in large hybrid vehicleapplications, such as hybrid locomotives, have the following needs forimprovement to enable them to meet performance expectations:

-   -   significantly improve hybrid cycle-life as measured by        ampere-hour throughput;    -   reduce internal ohmic resistance;    -   eliminate electrolyte stratification    -   maintain homogeneity of the electrochemistry across the surface        of the plates    -   significantly improve heat transfer from the plates to case        walls where it can be efficiently removed; and    -   significantly improve grid structural support for high        vibration/shock applications.

There thus remains a need for a large, high capacity electrochemicalenergy storage device that significantly reduces the baseelectrochemical, electrical, thermal, and mechanical conditions thatlead to premature capacity loss and abbreviated cell lifetime.

SUMMARY

These and other needs are addressed by the various embodiments andconfigurations of the present invention which are directed generally toelectrochemical energy storage devices and particularly to a largecapacity electrochemical cell that is substantially optimized for a dutycycle typical of hybrid locomotives in yard and/or road switchingservice and other hybrid vehicles.

In a first embodiment of the present invention, an electrochemicalenergy storage device is provided that includes:

(a) stacked electrodes arranged in electrode pairs, each electrode pairincluding an adjacent positive and negative electrode plates separatedby a layer or separator matrix of electrolyte and the electrode platepairs being arranged such that adjacent electrodes have opposingpolarities;

(b) a positive bus bar interconnecting the positive electrodes; and

(c) a negative bus bar interconnecting the negative electrodes.

In the device, one or more of the following is true at charging ordischarging rates between about 0.5 C and 2 C;

(c1) at least one of the positive and negative bus bars contacts thecorresponding positive and negative electrodes, respectively, to providea substantially uniform current density between each of thecorresponding contacted electrodes (e.g., the current flow between thecorresponding positive and negative electrodes preferably varies no morethan about 20%, more preferably no more than about 15%, and even morepreferably no more than about 10%);

(c2) at least one of the positive and negative bus bars contactsphysically at least half a length of a peripheral edge of each of thecorresponding positive and negative electrode plates, respectively, tomaintain a relative orientation of the bus bar and correspondingelectrodes substantially constant over time;

(c3) for each electrode plate pair, an electron travels an electricalcurrent path of a substantially constant electrical resistance, thecurrent path extending from the positive bus bar, through the positiveelectrode, traversing the electrolyte and through negative electrode,and to the negative bus bar;

(c4) a substantial length of a peripheral edge of at least one of thepositive and negative bus bars contacts physically a case enclosing theelectrode pairs to remove thermal energy generated by the flow ofelectricity;

(c5) for each electrode plate pair, a substantially constant electricalpotential gradient normal to the bus bars exists across any gridstructure on the surface of the electrodes (e.g., the electricalpotential gradient normal to the bus bars preferably varies no more thanabout 20%, more preferably no more than about 15%, and even morepreferably no more than about 10%);

(c6) at any point along the lengths of the positive and negative busbars, a substantially constant electrical potential exists betweenopposing points on the bus bars (e.g., the electrical potential betweenopposite points on the bus bars preferably varies no more than about20%, more preferably no more than about 15%, and even more preferably nomore than about 10%);

(c7) for each electrode plate pair, a substantially constantelectrochemical potential exists between any opposing points on theelectrode pair; and

(c8) at any point in an enclosed volume of the device, a substantiallyconstant electrochemical reaction exists. The above properties of thepresent invention hold true preferably at charging or discharging ratesbetween about 0.5 C and 2 C but, as can be appreciated, can also holdtrue at charging or discharging rates outside this range.

In another embodiment, an electrochemical energy storage device isprovided that includes one or more of the following features:

(1) the positive and negative bus bars are segmented, each segmentphysically contacting a corresponding electrode, the positive bus barsegments being stacked to define a plurality of spaced apart positiveelectrodes and the negative bus bar segments to define a plurality ofspaced apart negative electrodes, each of the negative electrodes beingreceived in a corresponding inter-electrode space between adjacentpositive electrodes and each of the positive electrodes being receivedin a corresponding inter-electrode space between adjacent negativeelectrodes;

(2) the positive and negative bus bars are segmented, each segmentphysically contacting most, if not all, of a selected peripheral edge ofa corresponding electrode, the adjacent segments being stacked one ontop of the other to form the respectively polarized bus bar;

(3) the positive and negative bus bars define one or more gas ventingspaces positioned between the positive and negative bus bars, the caseforming an interference fit along the peripheral edges of the bus bars;

(4) the stacked electrodes are compressed and maintained in compressionby a compression member separate from the case; and

(5) the case includes one or more offset member to define a channelbetween adjacent cases when the cases are positioned side-by-side.

According to an aspect of the present invention, the energy storagedevice, or cell, of the present invention is designed for partialstate-of-charge use and for a large ampere-hour capacity cell withimproved power, heat transfer and life characteristics compared toconventional cells. The cell cross-section can be any shape with squareor rectangular being preferred. The length can be changed to provide ascalable capacity (400 to 2,500 ampere-hours) using the same parts. Morespecifically, the present invention may be designed for:

-   -   uniform use of cell electrochemistry (long HEV life);    -   improved heat transfer from plates to case wall;    -   high power output due to lower ohmic resistance;    -   terminal designed and positioned to minimize longitudinal or        lateral cell interconnect pattern;    -   high vibration/shock capability (direct grid to case wall        support, electrode compression independent of case);    -   number of unique parts, independent of size scale of the cell;    -   200 to 4,500 amp hour capacity, scalable by a change in length;    -   unique geometry using extruded case and internal electrode        compression plates;    -   terminals at opposite ends for reduced interconnect length;    -   compression of electrodes by internal compression plates and        tension band; and/or    -   unique grid, bus bar and terminal arrangement.

In the following descriptions, lead-acid chemistry will be used toillustrate the invention. However, the principles illustrated areapplicable to other electrochemical cell chemistries such as forexample, nickle-metal hydride; nickle-zinc; and lithium ion.

The present invention can be an HEV specific cell design and a scalablecell that has been designed to improve power, heat transfer and lifecharacteristics compared to other large capacity lead-acid cells.

In a preferred embodiment, a cell is fabricated by stacking a number ofpositive and negative plate elements where the positive and/or negativeplate elements have a separator material between them. The positiveplate elements include a grid which is intimately connected to a sidestructure which forms a portion of the positive electrode (integralenlarged perimeter current collector) and a portion of the cellsidewall. The negative plate elements include a grid which is intimatelyconnected to a mirror image side structure which forms a portion of thenegative electrode and a portion of the opposite cell sidewall.

The cell of the present invention can be used with a separator materialimpregnated with a liquid electrolyte or it can utilize a gelelectrolyte.

The cell can have a number of advantages. By way of example, in the cellcurrent can flow in a substantially identical pattern across each plateat approximately a uniform current density to a low resistanceelectrode/terminal. This minimizes internal plate resistance andmaintains a substantial uniformity of current flow for all plates in thestack. The cell geometry can also allow heat energy to be generateduniformly by each plate, no matter where it is in the stack, and a majorportion of the internally generated heat directed to a side plate whichis in intimate contact with the inside of the plastic case.Additionally, the integral perimeter current collector structure alongwith active compression of the plate stack can impart to the cell astructural rigidity that resists damage caused by shock and vibration.

These and other advantages will be apparent from the disclosure of theinvention(s) contained herein.

The above-described embodiments and configurations are neither completenor exhaustive. As will be appreciated, other embodiments of theinvention are possible utilizing, alone or in combination, one or moreof the features set forth above or described in detail below.

The following definitions are used herein:

An electrochemical cell as used herein is a device that converts energyfrom an electrochemical reaction to useable electrical energy and inwhich the electrolyte is substantially stationary with respect to thepositive and negative electrode plate pairs.

A cell as used herein is an individual valve-regulated unit comprised ofone or more internal plate assemblies, each plate assembly including anegative plate, a separator material containing an electrolyte and apositive plate. The plate assemblies are all electrically connected inparallel such that the open-circuit voltage of the cell is substantiallythe same as the open-circuit voltage across any of the plate assemblies.The cell may have one or more external negative and positive terminals.

A battery as used herein is an individual electrochemical unit comprisedof two or more cells where the cells are electrically connected inseries or combinations of series and parallel. The battery may have oneor more external negative and positive terminals.

A valve regulated cell or battery is one in which internally generatedgas pressure causes a vent to open when a selected internal pressure isreached but does not allow reverse flow of gas into the cell or batteryfrom the outside.

A flow battery is a battery where the electrolyte is allowed to flowfrom a first storage container, between the plates of the cell and to asecond storage container. Since the electrolyte is in motion withrespect to the positive and negative electrode plate pairs when the cellis being charged or discharged, it is not an electrochemical cell asused herein.

A fuel cell is an electrochemical energy conversion device differingfrom an electrochemical cell as used herein in that it is designed forcontinuous replenishment of the reactants consumed. It produceselectricity from an external supply of fuel and oxygen as opposed to theself-contained electrolyte of an electrochemical cell. Additionally, theelectrodes within an electrochemical cell react and change as the cellis charged or discharged, whereas the electrodes of a fuel cell arecatalytic and relatively stable.

A capacitor is an electrical energy storage device that stores energy inthe electric field between a pair of closely spaced conductor plates.

C-rate: The charge and discharge current of a battery is measured inC-rate. A battery rated at 1 C means that a 1,000 amp-hour battery wouldprovide 1000 amps for one hour if discharged at 1 C rate. The samebattery discharged at 0.5 C would provide 500 amps for two hours. At 2C, the 1,000 amp-hour battery would deliver 2,000 amps for 30 minutes.

A battery rack is a mechanical structure in which cells are mounted.

A battery module is a collection of cells mounted in a battery rackframe assembly of convenient size.

A battery pack is an assembly of many individual cells connectedelectrically. The assembly may be include subassemblies or modulescomprised of individual cells. The battery pack usually, but not always,has one overall positive and negative terminals for charging anddischarging the cells in the pack.

Float service as applied to a battery means operating the battery underrigid voltage conditions to overcome self-discharge reactions whileminimizing overcharge and corrosion of the cell's positive grid.

Charge and Discharge Rates are commonly measured as a fraction ormultiple of the nominal ampere-hour capacity of the cell, C. Forexample, a C/2 charge rate is a charge rate of half the nominalampere-hour rating and a 10 C discharge rate is a discharge rate of 10times the nominal ampere-hour rating.

As used herein, “at least one”, “one or more”, and “and/or” areopen-ended expressions that are both conjunctive and disjunctive inoperation. For example, each of the expressions “at least one of A, Band C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “oneor more of A, B, or C” and “A, B, and/or C” means A alone, B alone, Calone, A and B together, A and C together, B and C together, or A, B andC together.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a typical prior art large capacity cell showingtypical current flow path. This is prior art.

FIG. 2 is a schematic of a large capacity cell of the present inventionshowing typical current flow path.

FIG. 3 is a schematic of a typical prior art large capacity cell showingtypical heat flow path. This is prior art.

FIG. 4 is a schematic of a large capacity cell of the present inventionshowing heat flow path.

FIG. 5 shows a schematic representation of an electrode platedeformation mechanism. This is prior art.

FIG. 6 is an isometric view of a grid structure of the present inventionshowing its integral bus bar-side plate segment.

FIG. 7 is an isometric view of nine stacked grid structures.

FIG. 8 is an isometric view of two grid assemblies with separators.

FIG. 9 is an isometric view of a positive plate-separator-negative platestack assembly.

FIG. 10 is an isometric view of a plate stack assembly with end plates.

FIG. 11 is an isometric view of a plate stack assembly strapped incompression.

FIG. 12 is an isometric view of a plate stack assembly prior toinstallation in a case.

FIG. 13 is an isometric exploded view of some interior components.

FIG. 14 is an isometric view of a case for the internal components.

FIG. 15 is an isometric exploded view of a cell of the presentinvention.

FIG. 16 is an isometric view before final assembly.

FIG. 17 is an isometric view of several components.

FIG. 18 is an isometric view of the cell after final assembly.

FIG. 19 is an isometric view of two cells in longitudinal arrangement.

FIG. 20 is an isometric view of several cells in lateral arrangement.

FIG. 21 is an isometric view illustrating current flow through a platepair.

DETAILED DESCRIPTION

The cell of the present invention has been designed specifically forpartial state of charge use. It is designed for a large ampere-hourcapacity cell with improved power, heat transfer, resistance to shockand vibration and lifetime characteristics compared to prior art cells.The cell cross-section is preferably approximately square or rectangularfor efficient space unitization in a large battery pack. The length canbe changed to provide a scalable capacity (from approximately 200 toapproximately 4,500 ampere-hours) using the same parts. The followingdescribes some of the principal features of the cell:

The cell is preferably designed for:

-   -   uniform use of cell electrochemistry (long HEV life);    -   improved heat transfer from plates to case wall;    -   high power output due to lower ohmic resistance;    -   terminal designed and positioned to minimize longitudinal or        lateral cell interconnect pattern;    -   high vibration/shock capability (direct grid to case wall        support, electrode compression independent of case);    -   same number of unique parts, independent of size scale of the        cell;    -   approximately 200 to approximately 4,500 ampere-hours capacity,        scalable by a change in length;    -   unique geometry using extruded case and internal electrode        compression plates;    -   terminals at opposite ends for reduced interconnect length;    -   compression of electrodes by internal compression plates and        tension band;    -   unique grid, bus bar and terminal arrangement; Specific features        of the cell of the present invention:    -   approximately equal electric resistance for all current paths        from the positive bus bar to the negative bus bar—this is        accomplished by opposing grid current collection and is designed        for benefits in partial state-of-charge operation, power and        usable energy;    -   large grid perimeter current collection and heat transfer        plates—the grid is formed with an integral enlarged perimeter        current collector to replace the conventional grid tab. The        integral enlarged perimeter current collector provides grid        structural support directly to the case walls;    -   extruded plastic case (e.g. ABS or polypropylene). Design to        facilitate structural integrity and provide capacity scalability        by changes in cell length;    -   minimum part count—a change in capacity is accomplished by a        change in length of the extruded case, all other parts the same;    -   terminal placement to minimize cell interconnect length. Dual        terminal configuration placed on the ends of the cell;    -   pressure relief valve placement to facilitate cell longitudinal        axis rotation—end to end connections; and    -   internal end plates with tension strap to maintain plates        compression independent of plastic case.

The design of the present invention achieves these goals in part by:

-   -   uniform current density between electrodes by “cross flow”        design (typically large capacity cells utilize a “U-flow”        design);    -   uniform use of electrode active material leads to a longer        electrode pair life in HEV application;    -   the bus bar current travels an identical length regardless of        plate location in cell stack; and    -   uniform use of all electrode pairs means a longer cell life.

The benefits of this approach include:

-   -   cell design is expected to significantly improve hybrid        cycle-life (amp hour throughput);    -   significant reduction in ohmic resistance;    -   significant improvement in heat transfer; and/or    -   significant improvement in grid structural support for high        vibration/shock applications.

As can be appreciated, the cross-section of the cell can be cylindricalor elliptical. In these cases, the length of the cell can still bechanged to provide a scalable cell capacity. In these cases, the currentdensity remains substantially uniform between opposing electrodes and asubstantially equal electric resistance should exist for all currentpaths from the positive bus bar to the negative bus bar. In addition,the heat dissipation advantages and the structural advantages of thecell will remain.

In the following descriptions, lead-acid chemistry will be used toillustrate the invention. However, the principles illustrated areapplicable to other electrochemical cell chemistries such as forexample, nickle-metal hydride; nickle-zinc; and lithium ion.

Current Flow Distribution

FIG. 1 is a schematic of a typical prior art large capacity internalcell construction showing the approximate path of current flow on theplate surfaces. This figure illustrates a stack 102 of three negativeplates and three positive plates where the positive plates are encasedin separators. In this example, arrows 107 indicate the flow of currentalong a positive plate 101. The current flows at ever-increasing currentdensity towards a tab 106 which is connected to a positive bus bar 103,the bus bar 103 having, in this example, two terminals 105. All thecurrent along the plate 101 flows through the tab 106 and along the busbar 103 to the terminals 105. In this example, the positive/negativeplate pairs are electrically in parallel. The current flow in thenegative terminals, negative bus bar, negative tabs and negative platesis similar but in the opposite direction to their positive counterparts.Current flows from the negative plates across the electrolyteimpregnating the separator material to its neighboring positive plates.As can be seen, the current density is highest nearest the tab 106 andcurrent flow direction varies significantly over the surface of theplate. This typically causes the electrode surface near the tabs of theplate to deteriorate at different rates (usually at a higher rates) thanthe electrode surface at larger distances from the tabs.

FIG. 2 is a schematic of a large capacity cell internal construction ofthe present invention showing the approximate path of current flow onthe plate surfaces. This figure illustrates a stack of eight positiveplates and nine negative plates where the positive plates are encased inseparators. Both positive and negative plates are formed by grids withintegral enlarged perimeter current collector segments 202 where afurther enlarged portion 203 forms a bus bar segment. In this example,arrows 204 indicate the flow of current along a positive plate. Thecurrent flows across the electrode plates along paths, that areapproximately perpendicular to the bus bars 201 and 202, and then alongthe perimeter current collector to a terminal (shown in a later figure)inserted into the end of the enlarged section. In this example, thepositive/negative plate pairs are electrically in parallel. The currentflow in the negative grid structure and perimeter current collector issimilar but in the opposite direction to its positive counterparts.Current flows at approximately uniform current density from the negativeplate across the electrolyte impregnating the separator material to itsneighboring positive plate. As can be seen, the direction of currentflow is approximately uniform over the entire plate. This allows theelectrodes, which include the grid and paste material, to change overtime at a reasonably constant rate over each electrode's entire surfacearea, tending to extend the life of the plates and hence the cell.

Internal Heat Flow Distribution

FIG. 3 is a schematic of a typical prior art large capacity cellinternal construction showing the approximate paths of heat flow on theplate surfaces. This heat is the heat generated by the ohmic resistivelosses in the electrolyte and plates of the cell. This figureillustrates a stack 301 of three negative plates and three positiveplates where the positive plates are encased in separators. In thisexample, arrows 302 indicate the flow of heat along either a positive ornegative plate. If the sides of the plates are close to the insides ofthe case walls (not shown), then heat can traverse the gap between theedge of the plates and the case wall. If the case walls are sufficientlythin, then heat can flow through the case walls to the outside where itcan be removed, for example, by a forced convection cooling system.Typically, only a small fraction of heat flows out the bottom of thecell because the cell normally rests on a non-conductive surface toisolate the cell from electrical ground faults. Also, a small fractionof heat flow may follow the path of the tabs and bus bar. Little heatwill be removed through the top of the case because of an air gap abovethe plate stack that absorbs gas vented from the cell reactions,especially during overcharging. Some heat may flow across the plates(orthogonal to arrow 302) but the lowest net resistance to heat flow iscommonly across the plates as shown by arrow 302.

FIG. 4 is a schematic of a large capacity cell of the present inventionshowing the approximate paths of heat flow on the plate surfaces. Thisfigure illustrates a stack 401 of nine negative plates and eightpositive plates where the positive plates are encased in separators. Asin FIG. 2, both positive and negative plates are formed by grids withintegral enlarged perimeter current collector segments where a furtherenlarged portion forms a bus bar segment. In this example, arrows 402indicate the flow of heat along either a positive or negative plates.The flow of heat is primarily along the plate towards the integralenlarged perimeter current collector. A small amount of heat can flowaway from the integral enlarged perimeter current collector but it hasto flow across a resistive gap formed by separator material andelectrolyte to reach the integral enlarged perimeter current collectorof the adjacent plate of opposite electrical polarity. In thisconfiguration, the sides of the perimeter current collectors areintimately in reasonably intimate contact with the insides of the casewalls (described in subsequent figures). In this configuration, the heatflow can readily traverse the contact area between the side of theperimeter current collector and the inside of the case wall. The casewalls are made sufficiently thin so that heat can flow efficientlythrough the case walls to the outside where it can be removed, forexample, by a forced convection cooling system. Typically, only a smallfraction of heat flows out the bottom of the cell because the cellnormally rests on a non-conductive surface to isolate the cell fromelectrical ground faults. Also, a small fraction of heat flow may followthe path of the enlarged bus bar segment. Little heat will be removedthrough the top of the case because of an air gap above the plate stackthat absorbs gas vented from the cell reactions, especially duringovercharging. Some heat may flow across the plates (orthogonal to arrow402) but the lowest net resistance to heat flow is commonly along theplates as shown by arrow 402.

Structural Integrity

FIG. 5 shows a schematic representation of a plate deformation mechanismfor the construction of prior art cells such as shown in FIGS. 1 and 3.This mechanism may be enabled by application of a strong force such asintense vibration or shock loading caused by dropping for example or byprolonged vibration such as experienced by a locomotive moving along thetracks. In many prior art large capacity batteries, the cells arefabricated by stacking a series of positive and negative platesseparated by a separator material. Next, positive and negative bus barsare then typically welded to positive and negative tabs that extend fromthe tops of the positive and negative plates respectively, as shown forexample in FIGS. 1 and 3. The tabs for the negative plates are typicallylocated off to one side of the plate while the tabs for the positiveplates are located off to the opposite side. This positioning, which isshown for example in FIGS. 1 and 3, allows the bus bars to be attachedso that positive and negative terminals are sufficiently far apart toavoid incidental electrical shorting. The bus bars therefore hold thepositive and negative plates in the desired position with the remainderof the stacked structure held in position by friction between the platesand separator material. An extra negative plate may be added on the endof the stack so that the negative bus bar, when attached to all thenegative plates, allows the negative plates on the ends of the stack tohold the assembly together to a necessary extent to allow installationinto a battery case. Next, the stacked assembly is typically positionedtightly inside a battery container case. The battery case thereforeholds the stacked assembly in the desired position where now the insidewalls of the battery case, again aided by friction between the platesand the separator material and by the clamping action of the bus bars,secure the plates and separator layers in the stacked assembly frommoving relative to one another. Finally, the separator material isimpregnated with an appropriate electrolyte and the top of the batterycase is installed. FIG. 5 shows a typical plate 501, which may bepositive or negative, and its electrode tab 502 offset to one side ofthe top of the plate 501. When the plate 501 is welded to its bus bar,the plate becomes mechanically attached to the bus bar. However, exceptfor frictional forces between the plates and the separator material, allplates can rotate about an axis 503 that is approximately coincidentwith its tab 502. In the case of severe and prolonged vibration or shockloading, the net effect of the changing gravity and frictional forcesmay be to cause a plate such as 501 to rotate about an axis such as 503.The rotation may occur as a result of the tab being deformed which is alikely mechanism for a material such as lead. The corner 505 of theplate 501 can rotate downward by a small amount causing the plate 501 tocome to a new position shown by the a new plate position 504. Thisamount of plate movement can result in a significant change in theresistance between adjacent positive and negative plates since theseparator material is generally compressible and even more deformablethan the plate material and will change its shape and volume to adjustto the new plate position. It is noted that, when electrolyte is addedto complete the fabrication process of the battery, that the frictionbetween the plates and separator material is generally reduced.

FIG. 6 is an isometric view of a grid structure of the present inventionshowing its integral enlarged perimeter current collector segment. FIG.6 illustrates a typical positive or negative plate which consists of agrid section 601 on which the appropriate positive or negative paste isapplied. Also shown are an integral enlarged perimeter current collectorsegment 602 which has a further enlarged portion 603 which functions asa bus bar and a top and bottom sub-segment 604 which provides structuralrigidity. The segments 601, 602, 603 and 604 are preferably all part ofa single cast lead structure.

Cell Design

FIG. 7 is an isometric view of a stack 701 of nine grid structures, eachgrid structure 702 identical to the structure shown in FIG. 8. Thisfigure illustrates how the enlarged portions 703 stack together to forma bus bar. This enlarged portion is preferably rounded in cross-sectionbut may be elliptical, slightly rectangular or square in cross-section.This figure also illustrates how the side sections 704 of the perimetercurrent collector, stack together to form a side plate. As can be seen,the top and bottom sub-segments 705 stack together to give the structurerigidity and help maintain proper separation between plates.

FIG. 8 is an isometric view of two grid assemblies with separators. FIG.8 shows separator pockets 802 which are slipped over plates 801. Theseparators 802 fit closely along the inside of the raised portions ofthe perimeter current collector which comprises a side plate sub-segment803 and top and bottom sub-segments 805. Typically and preferably, theseparator pockets encase the positive plates but alternately theseparators be used to encase negative plates. In some cases it may bedesirable to use thin separator pockets to encase both positive andnegative plates.

FIG. 9 is an isometric view of a positive plate-separator-negative platestack assembly illustrating how a negative stack 901 comprised of nineplates is interlaced with a positive stack comprised of eight plates 902to form a cell, where all plate assemblies are connected electrically inparallel. The top and bottom sub-segments 904 and 905 of the perimetercurrent collectors form a rigid structure and provide space 906 alongthe top and bottom for compression assembly (described later) and gasventing. In this figure, a negative grid 903 is shown facing outwards.

FIG. 10 is an isometric view of a plate stack assembly with end plates.The end plates 1001 and 1002 complete a stack of positive and negativeplates. The end plates contain a groove 1003 which allows a compressionstrap to tie the assembly together in positive compression. The groove1003 also has sufficient clearance to allow gas venting as will bedescribed in a subsequent figure. The end plates 1001 and 1002 alsoinclude a passage 1004 in the enlarged section that lines up with theenlarged bus bar segments of the grid plates. This provides forinsertion of an electrical terminal as described in subsequent figures.

FIG. 11 is an isometric view of a plate stack assembly with end plateswhich are strapped in compression. The end plates 1101 and 1102 hold thestack in compression with strap 1103. This strap may be elastic or havesome other means of tightening (not shown) so as to maintain the platestack within a desired range of compression (typically the cells wouldbe maintained under positive compression force equivalent to a pressureof about 10 to about 100 kPa, depending on the strength of the separatormaterial. The compression is meant to be high enough to prevent activepaste particles from dislodging and moving around while not being sohigh that the separator matrix is distorted or electrolyte is squeezedfrom the separator matrix). The strap 1103 may be designed to maintainthe plate stack within a desired range of compression as the platesarid/or separators expand and contract slightly with temperature, levelof charge, discharge or charging episodes. This positive compression isknown to extend cell lifetime as it prevents movement and sloughing ofpaste material on the positive and negative plates. In many prior artlarge energy storage cells, compression of the plate stack is oftenobtained by forcing the stack into a case and relying on the case tomaintain compression. This method has no provision for maintainingcompression on the stack when the stack shrinks relative to the interiorcase walls.

FIG. 12 is an isometric view of a plate stack assembly with end capsprior to installation in a case. End caps 1201 and 1202 are added to thestack and are positioned on the end plates described in FIG. 11. The endcaps 1201 and 1202 contain openings 1203 for gas vents on both the topand bottom of the end caps 1201 and 1202. The end caps 1201 and 1202 donot provide compression for the stack but do form the outside ends ofthe cell. As can be seen, the end caps include a passage 1204 in theenlarged section that lines up with the enlarged bus bar segments of oneset of grid plates on one side but no passage on the opposite side 1205.In this example, the passages 1204 on the right front side will containthe negative cell terminals while the passages (not shown) on the leftback side will contain the positive cell terminals.

FIG. 13 is an isometric exploded view of interior components for furtherreference. This view shows a negative plate 1302, a positive plate 1303and its separator pocket 1304, and an end plate 1305. A stack 1301 withsome of these components assembled is also shown.

FIG. 14 is an isometric view of a case 1402 for containing the internalcomponents. The case 1402 is an integral plastic container molded toprovide for the perimeter current collectors of the stacked plateassembly. The case in this example is shown with molded sections 1401for the bus bar assemblies and other molded sections 1403 to allow forgas vents. As can be seen, the case can be made longer to accommodate alarger stack of plates. The ability to readily scale the storagecapacity of the cell by making the cell longer is an important featureof the present invention as it allows the cell to be scaled up or downin electrical storage capacity by changing only the case length. As canbe appreciated, the shape of the molding can be changed for differentgeometries of perimeter current collectors arid different aspect ratiosof the plate widths and heights.

FIG. 15 is an isometric exploded view showing most of the components ofthe cell of the present invention. FIG. 15 shows internal componentssuch as a negative grid 1502, a positive grid 1503 and a separator 1504.These are stacked together as shown by an assembly 1507 and the stackheld together in compression by end plates 1505 and strap 1506, asdescribed previously. The strapped stack along with its end caps 1506are fit into the case 1501 as will be described subsequently. Then,components such as vents 1511, vent plugs 1512 and electrode terminals1513 are installed as also will be described subsequently.

FIG. 16 is an isometric view showing a case 1601 and a completed stackassembly 1602 aligned for final assembly. As will be describedsubsequently, the case 1601 may be expanded while the stack assembly1602 is inserted so that, after insertion, the case 1601 contracts andforms a tight interference fit around the stack assembly 1602.

Vent Configurations

FIG. 17 is a more detailed isometric view of several components such asa vent port 1702 with its vent hole 1703. Vent ports are typicallyinstalled in the top vent opening on one side of the cell and in thebottom vent opening on the opposite side of the cell as shown forexample in FIG. 16. The vent ports are designed with a pressure reliefmeans to seal the cell below a first predetermined pressure (typicallyin the range of 1 psi or less) and to open the vent port above a secondpredetermined pressure (typically in the range 1 to 3 psi or greater).The pressure relief means may be a Bunsen valve or another low-costvalve arrangement that is compatible with the fumes associated with thegas under pressure. There are a number of prior art means of pressurerelief mechanisms known for sealed cells. A vent plug 1701 is used toclose off the unused vent openings as also shown in FIG. 16. Anelectrode terminal 1704 with its recessed connection port 1705 is alsoshown. In a lead-acid cell, the vent port 1702 and vent plug 1701 aretypically made of a material such plastic (for example ABS orpolypropylene) that is resistant to corrosion or attack by theelectrolyte. The electrode terminal 1704 is typically made of aconductive metal such as lead, copper, aluminum or steel or a compositeof these materials.

The principal purpose of a vent valve is to release pressure duringover-charge. A vent valve is typically designed to start relievingpressure at approximately 0.5 to 3 psi and to pass an amount of gas thatis greater than would be expected from electrolysis at the end ofcharging cycle. The issue for fast charging or hybrid operation is thatan individual cell or module could electrochemically fail and then gointo electrolysis and a subsequent boiling condition upon theapplication of maximum current. The problem is that under the abovecondition typical cell gas vent valves can not pass enough gas and thepressure goes up. As the temperature goes up, the case distorts andfinally if the condition persists the case fails along some edge orseam. Often the cell in a HEV application can go some time before it isrecognized as having a problem.

A solution would be to have the normal relief valve and an over-pressurerelief plug or burst disk. Under normal conditions the relief plug doesnothing. When an over-pressure occurs the over-pressure relief plugblows out. This relieves the over pressure condition and would bedesigned to give a clear visual indication that the cell has experiencedan over pressure condition and must be removed from the pack.

With the cell of the present invention, another solution may be to havetwo low pressure relief valves, one on top of a first end cap and one onthe bottom of a second end cap. Additionally, there would be two highpressure relief valves, one on the bottom of the first end cap and oneon the top of the second end cap. The low pressure relief valves may beset to vent gas when the pressure exceeds a first predetermined level(typically in the range of about 0.5 psi to about 5 psi). When thepressure is reduced below this pressure range, the low pressure reliefvalve closes. The high pressure relief valves may be set to vent gaswhen the pressure exceeds a second predetermined level (typically in therange of about 5 psi or higher). In addition, the high pressure reliefvalves may have a substantially larger orifice than the low pressurerelief valves. When either of the high pressure valves are activated,they may be constructed to remain open, and/or sound an alarm on a cellmonitoring system, if available.

Cell Construction Method

FIG. 18 is an isometric view of the cell after final assembly showingthe case 1801 and one of the two end caps 1810. A gas vent port 1802 isshown installed in the upper gas vent molded opening of the end cap1810. A vent plug 1803 is shown installed in the lower gas vent moldedopening of the end cap 1810. In the opposite end cap (not visible inthis figure), a gas vent port could be installed in the lower gas ventmolded opening and a vent plug could be installed in the upper gas ventmolded opening. This would allow gas to be vented from either the upperor lower volumes (see volume 906 in FIG. 9) independent of the up ordown orientation of the cell. Alternately, four vent ports could beinstalled in all four gas vent molded openings. Electrode terminals 1804are shown installed in the molded terminal openings. Electrode terminalsof opposite polarity 1805 are shown installed in the molded terminalopenings on the opposite end of the cell.

The present invention is an HEV specific cell design and is a scalablecell that has been designed to improve power, heat transfer and lifecharacteristics compared to other large capacity cells. An importantfeature of the present invention is the ability to easily fabricatelarger capacity cells by increasing the stack size (as determined by thenumber of positive/negative plate pairs) and lengthening the cell case.This capacity scaling is possible while maintaining a desired endcross-section. This means that a battery pack can use the same racksystem but with different cell counts (and hence overall series stringvoltage). This allows a trade off between pack voltage and capacity.

In a preferred embodiment, a cell is fabricated by stacking a number ofpositive and negative plate elements where the positive and/or negativeplate elements have a separator material between them. The positiveplate elements include a grid which is intimately connected to a sidestructure which forms a portion of the positive electrode (integralenlarged perimeter current collector) and a portion of the cellsidewall. The negative plate elements include a grid which is intimatelyconnected to a mirror image side structure which forms a portion of thenegative electrode and a portion of the opposite cell sidewall. The cellof the present invention can be used with a separator materialimpregnated with a liquid electrolyte or it can utilize a gelelectrolyte.

The following is a step by step description of the general order of cellfabrication.

-   1. The cast electrode grids (positive and negative) are made in a    conventional manner but with a thicker edge on three sides to form    an enlarged perimeter current collector border. The edge or    perimeter current collector preferably forms a three-sided enlarged    cross-section in plan view of plate.-   2. The cast electrode grids also employ large over sized rounded    corners (preferably round, less preferably square).-   3. The grids are pasted in a normal fashion to become electrodes. A    paste-like mixture of lead oxide, sulfuric acid and water is applied    to the positive grids. A paste-like mixture of lead oxide, sulfuric    acid, water and expander is applied to the negative grids.-   4. The positive and negative electrodes are then assembled with    separators into a stack. The enlarged perimeter current collectors    of the respective positive and negative grids are arranged on    respective sides. The additional edge thickness of the enlarged    perimeter current collectors on the grids essentially provides the    spacing for the opposite polarity electrode and separator and    provides structural rigidity to the final stacked assembly.-   5. The stack is compressed and held by nonconductive end plates and    strapping.-   6. Using a hot plate technique the enlarged perimeter current    collectors are all welded together on the respective positive and    negative sides, forming a stack that is now electrically connected    has substantial structure integrity.-   7. End terminals are prepared at the respective ends of the stack    for the enlarged bus bar sub-segments of the perimeter current    collectors, two positive and two negative terminals at opposite ends    of the stack.-   8. The stack is then slide longitudinally into an extruded ABS case    that is pre-heated to expand (mechanical expansion of the case is    another option).-   9. The extrusion case has over sized corners, allowing the oversized    corner of the now welded grids to key into place-   10. As the preheated extruded case cools it forms a slight    interference fit along the now-welded side sections of the perimeter    current collectors of the stack to provide for efficient heat    transfer to the appropriate case walls.-   11. The extrusion is arranged to provide a gas venting space on at    least two longitudinal sides of the cell stack.-   12. Plastic end caps are mounted and sealed by glue (prior art    technique).-   13. Terminals are also sealed by color coded glue (prior art    technique).-   14. Integral to the end caps are relief valves that align with the    gas venting spaces.-   15. Electrolyte is inserted via the relief valve holes and    distributes over all electrodes (prior art technique).-   16. Relief valves are assembled on the end caps completing the cell.

A high capacity cell of the present invention has a width in the rangeof about 100 mm to about 300 mm; a height in the range of about 200 mmto about 500 mm; and a length in the range of about 300 mm to about 800mm. The cell has a mass in the range of about 20 kg to about 400 kg. Thecell has an open-circuit voltage in the range of about 1 volt to about 5volts (depending on the cell chemistry) at the beginning of its lifecycle and an ampere-hour capacity in the range of about 200 ampere-hoursto about 4,500 ampere-hours at the beginning of its life cycle.

The grid structures are typically made from lead or lead alloys. Thegrid structure also comprises an integral perimeter current collectorcomprised of a side plate segment, a top and bottom segment and anenlarged portion which functions as a bus bar. These may be made of thesame material as the grid structure. The integral side plate segment,top and bottom segments and an enlarged portion may also be made ofother conductive metals or alloys such as, for example, aluminum orcopper or combinations of these metals to improve electrical and thermalconductivity. The enlarged portion which functions as a bus bar may alsobe hollowed out so that a more conductive metal core can be inserted tofurther reduce internal ohmic resistance. The end plates, end caps andcase may be fabricated from ABS, polypropylene, nylon or any otherelectrolyte-resistant plastic commonly used for battery cases. Theseparator material may be any commonly used separator material used inlead-acid batteries such as for example, absorptive glass mat,polypropylene loose weave cloth, hyalyte glass, daramic microporousfabric, electrolytic paper and the like.

The thickness of the grids is typically in the range of about 1 to 10 mmand the thickness of the separators is typically in the range of about 2to 12 mm. A cell of the present invention may contain from about 10 toabout 100 plate pairs.

Connections to Other Cells

FIG. 19 is an isometric view of two cells in longitudinal arrangement.Positive terminals 1901 are connected to negative terminals 1902 so thatthe cells are electrically connected in series. In this example, gasvent ports 1903 are shown on the top side of the cells and vent plugs1904 are shown on the bottom side of the cells for the end plates inview. As can be seen, the cells can be lined up in a compact seriesstring with couplers 1905 as shown. Alternately, positive terminals canbe constructed as male fittings to fit inside negative terminals whichmay be female. Alternately, the positive terminals may be coupled to thenegative terminals using compact coupling unions (not shown). As canalso be seen, the sides of the cells are recessed because of theenlarged bus bar channels top and bottom, so that when strings of cellsare arranged side by side (not shown), the recessed sides form passageways that can be used as convective air ducts for efficient cooling. Asnoted previously, heat preferentially flows from inside the cells alongthe positive and negative plates to their respective side plates whichare in intimate contact with the side walls of the cells.

When a series of cells are connected, they can be physically placed in alongitudinal or lateral pattern. When a series of cells are connected ina longitudinal pattern, they are in an axial position and alternatelyrotated 180 degrees to make the negative to positive terminals align asshown for example in FIG. 19.

FIG. 20 is an isometric view of several cells in lateral arrangement.Positive terminals 2001 are connected to negative terminals 2002 so thatthe cells are electrically connected in series. In this example, gasvent ports 2003 are shown on the top sides of alternate cells while ventplugs 2004 are shown on the bottom sides of alternate cells. As can beseen, the cells can be lined up in a compact string with positiveterminals being attached to negative terminals by short straps 2005 orspecially made U-shaped couplers (not shown). As can also be seen, thesides of the cells are recessed because of the enlarged bus bar channelstop and bottom, so that when cells are arranged side by side as shown,the recessed sides form passage ways that can be used as forced airducts for efficient convective air-cooling.

The straps, couplers or unions that are used to connect adjacent cellsmay be made from any suitable material such as, for example, copper,aluminum, lead or any combination of these.

Cell Resistance

The total resistance of a cell can be considered in 3 principal parts:(1) resistances in the bus bar structures; (2) resistances across theplate grids; and (3) resistances across the electrolyte betweenelectrode plates. Typically as the plates of a cell are made larger, therelative contributions of the resistances across the electrolyte betweenelectrodes decreases relative to the resistances along the plate gridstowards or away from the bus bars. In cells of the size required forlarge HEV applications, the resistances across the plate grids are ofthe same order as the resistances across the electrolyte betweenelectrodes so any reductions of the resistances across the plate gridsand resistance in the bus bar structures are of significance to overallcell resistance.

In the cell of the present invention, current flows at approximatelyuniform current density from the negative plates across the electrolyteimpregnating the separator material to its neighboring positive plates.Current flows along paths that are approximately perpendicular to thebus bars across the electrode This minimizes internal plate resistanceand maintains a approximately uniform current flow across theelectrolyte for all plates in the stack plates . This geometry alsoallows heat energy to be generated uniformly by each plate no matterwhere it is in the stack and directs a major portion of the internallygenerated heat to a side plate which can be in substantial or evenintimate contact with the inside of the plastic case. Additionally, theintegral perimeter current collector structure along with activecompression of the plate stack gives the cell of this invention astructural rigidity that resists damage caused by shock and vibration.The cell of the present invention incorporates “flow through” design tobalance the electrode usage throughout the cell and across each grid.This method directs the current such that no matter which active pairthe current crosses on, it travels the same total distance in the busbar. FIG. 21 is an isometric view illustrating this flow throughprinciple. FIG. 21 shows the current path for the component of currentgenerated by a single plate pair. The current component flows into thepositive terminal 2101 along a positive bus bar 2103, across and througha plate pair and along a negative bus bar 2104 and out the negativeterminal 2102. As can be seen, the component current path for any platepair will always involve the same length of bus bar. Because currentflows in an approximately identical and uniform pattern across eachplate, the current density of the current flow through the electrolytebetween the plates is also substantially uniform. This allows theelectrical potential at any point across the plates to remainsubstantially constant (the electrical potential being the open-circuitvoltage minus the resistive or IR drop). Thus, the electrochemicalreactions during either charging or discharging remain substantiallyuniform over the surface of the plates. This, in turn, allows thesurface chemistry to tend to change uniformly over the surface of theplates over the lifetime of cell operation. This uniformity ofelectrochemical minimizes the tendency to form areas on the plates oflower conductivity, higher sulfation and the like. It can also be seenfrom FIG. 21, that the voltage measured between the positive andnegative bus bars is substantially the same between opposite, oropposing, points anywhere along the length of the bus bars.

The following is a brief analysis of the internal cell resistance of alarge cell of the present invention. These are typical values for atypical configuration of the cell. The electrical resistivity (ρ) oflead (Pb) is approximately 0.207 milliohms-mm. To determine theresistive drop (R) in ohms, of a lead component requires the electricalresistivity to be multiplied by the length (L) and divided by the crosssection (A) of the component. Only 1 of the 2 terminals will beanalyzed. This results in the resistive value of the single path beingdivided by 2 to account for the other parallel path. Again, due tosymmetry, only half the path (positive terminal, positive bus bar, andpositive grid) will be analyzed. Multiplication of this value by twowill be required to determine the total resistance of the path.

The terminal may include, for example, a copper threading integrallylocated in the end of the bus bar. Due to copper's low electricalresistivity, the terminal distance can be neglected.

The bus bar consists is formed by the stack of the enlarged sub-sectionsof the perimeter current collector of each grid, the perimeter currentcollectors of each stack being welded together to form a lead rod.Additional length will be included to account for the lateral travelthroughout the cross-section of the perimeter current collectors(referred to subsequently as the “C” section).

The grid is encompassed on one complete and two partial sides by the “C”section. Thus the length of travel in the grid will be estimated as ½length of a grid side.

To find the total resistance of the cell, the above value must bemultiplied by 2 (to account for other symmetric side) and divided by 2(to account for parallel terminals). This becomes a total ohmicresistance value of the terminals, bus bars, and grid equal to 0.0386milliohms.

In a large prior art cell of comparable size and capacity as the cell ofthe present invention, the terminal, bus bar, and grid resistance limitthe rate of power from the active material. Under high power conditionsunequal uneven resistance across the active material can result inelectrode state-of-charge imbalance as a function of location.Typically, area and length are used to determine the ohmic resistance ofthe terminal, bus bar, and grid of a cell.

A conventional prior art cell may have 4 parallel paths for current totravel though (4 terminal entrance and exits). Only 1 path will beanalyzed and its result divided by 4 to account for the parallel paths.The current travels the following path: positive terminal, positive busbar, positive tab, positive grid, electrolyte (not accounted for),negative grid, negative tab, negative bus bar, negative terminal. Theresistive drop is therefore symmetric about the electrolyte. Thefollowing analysis will only account for ½ of the path and then bemultiplied by two to obtain the total resistive drop for the path.

The terminal goes through 3 distinct cross-sections as it makes its wayto the bus bar. Analysis will begin just below the threaded copperinsert (the same as used for the example analysis of the cell of thepresent invention), since copper's resistivity is 1/10th that of leadand therefore a minimal resistance.

The terminal center-taps the bus bar, meaning that the current flowingthrough the terminal can travel in two directions (outward) when itreaches the bus bar. These two paths are effectively in parallel.Analysis has shown that current reduces nearly linearly as it proceedsfrom the center of the bus bar to the end of the bus bar. It istherefore assumed that the average current travels a path length equalto ½ of the half-bus bar part (equivalent to ¼ of the specific terminalsbus bar length).

Current flows into the grid from the bus bar through a tab. The currentthen spreads out through the grid. Average length of travel within thegrid is approximated from grid design and size. There are 9 grids andtherefore 9 parallel paths for the current to flow.

To find the total resistance of the cell, the above value must bemultiplied by 2 (to account for other symmetric side) and divided by 4(to account for parallel terminals). This becomes a total Ohmicresistance value of the terminals, bus bars, and grid equal to R=0.0681milliohm

Thus, the cell of the present invention can reduce internal cell ohmicresistance of the electrode plates and current distribution system byapproximately a factor of two over that of a comparable prior art cell,due to the more efficient geometry of the plates with their integralperimeter current collectors. This represents a significant reduction intotal cell resistance for large cells based on the design principles ofthe present invention.

A number of variations and modifications of the invention can be used.As will be appreciated, it would be possible to provide for somefeatures of the invention without providing others. For example, in onealternative embodiment, a number of cells can be packaged into a singlemolded case to form a battery unit with a different output voltage. Inthe case of lead-acid cells, a number “n” of 2.1 volt cells can beconnected electrically in series and packaged in a single or in acomposite outer case molded to accept the individual cells to form an“n” times 2.1 volt battery. Additional cells can be added in this way aslong as provisions are made for gas venting of cells in the interior ofthe assembly. This configuration can be deduced from FIG. 19 where thecouplers 1905 are collapsed and replaced by a nonconductive separatorplate with provisions for internally connecting the bus bars of the twocells and with provisions for internal vent passages.

As can be appreciated by one skilled in the art, the stackable andscaleable geometry of the present invention can be adapted to flow cellsand flow batteries by adding suitable electrolyte tanks, pumps, controlvalves and electrolyte flow passages in the cells.

The present invention, in various embodiments, includes components,methods, processes, systems and/or apparatus substantially as depictedand described herein, including various embodiments, sub-combinations,and subsets thereof. Those of skill in the art will understand how tomake and use the present invention after understanding the presentdisclosure. The present invention, in various embodiments, includesproviding devices and processes in the absence of items not depictedand/or described herein or in various embodiments hereof, including inthe absence of such items as may have been used in previous devices orprocesses, for example for improving performance, achieving ease and\orreducing cost of implementation.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of theinvention are grouped together in one or more embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description, with eachclaim standing on its own as a separate preferred embodiment of theinvention.

Moreover though the description of the invention has includeddescription of one or more embodiments and certain variations andmodifications, other variations and modifications are within the scopeof the invention, e.g., as may be within the skill and knowledge ofthose in the art, after understanding the present disclosure. It isintended to obtain rights which include alternative embodiments to theextent permitted, including alternate, interchangeable and/or equivalentstructures, functions, ranges or steps to those claimed, whether or notsuch alternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

1. An electrochemical energy storage device, comprising: (a) a pluralityof stacked electrodes arranged in a plurality of electrode plate pairs,each electrode pair comprising an adjacent positive and negativeelectrode separated by a layer or separator matrix of electrolyte andthe plurality of electrode plate pairs being arranged such that adjacentelectrodes have opposing polarities; (b) a positive bus barinterconnecting the positive electrodes; and (c) a negative bus barinterconnecting the negative electrodes, wherein at least one of thefollowing is true at charging or discharging rates between about 0.5 Cand 2 C; (c1) at least one of the positive and negative bus barscontacts the corresponding positive and negative electrodes,respectively, to provide a substantially uniform current density betweeneach of the corresponding contacted electrodes; (c2) at least one of thepositive arid negative bus bars contacts physically at least half alength of a peripheral edge of each of the corresponding positive andnegative electrode plates, respectively, to maintain a relativeorientation of the bus bar and corresponding electrodes substantiallyconstant over time; (c3) for each electrode plate pair, an electrontravels an electrical current path of a substantially constantelectrical resistance, the current path extending from the positive busbar, through the positive electrode, traversing the electrolyte andthrough negative electrode, and to the negative bus bar; (c4) asubstantial length of a peripheral edge of at least one of the positiveand negative bus bars contacts physically a case enclosing the electrodepairs to remove thermal energy generated by the flow of electricity;(c5) for each electrode plate pair, a substantially constant electricalpotential gradient normal to the bus bars exists across any gridstructure on the surface of the electrodes; (c6) at any point along thelengths of the positive and negative bus bars, a substantially constantelectrical potential exists between opposing points on the bus bars;(c7) for each electrode plate pair, a substantially constantelectrochemical potential exists between any opposing points on theelectrode pair; and (c8) at any point in an enclosed volume of thedevice, a substantially constant electrochemical reaction exists.
 2. Thedevice of claim 1, wherein (c1) is true.
 3. The device of claim 2,wherein the positive electrodes are electrically connected in parallelwith the negative electrodes and wherein the at least one of thepositive and negative bus bars contacts physically the entire length ofa peripheral edge of the corresponding positive and negative electrodeplates, respectively.
 4. The device of claim 2, wherein theelectrochemical energy storage device is a cell and wherein the currentdensity between the corresponding positive and negative electrodesvaries no more than about 15%.
 5. The device of claim 1, wherein (c2) istrue.
 6. The device of claim 5, wherein the electrochemical energystorage device comprises a plurality of cells, wherein the electrodepairs are electrically connected in parallel, and wherein the casecontacts the at least one of the positive and negative bus barssubstantially along the entire peripheral edge of the bus bar.
 7. Thedevice of claim 6, wherein the plurality of stacked electrodes iscompressed and maintained in compression by a compression member.
 8. Thedevice of claim 1, wherein (c3) is true.
 9. The device of claim 8,wherein (c4) is true.
 10. The device of claim 1, wherein the at leastone of the positive and negative bus bars comprising a plurality ofsegments, wherein each segment is associated with a correspondingelectrode, wherein the bus bar segments are stacked one on top of theother, wherein each of the bus bar segments has a first width, whereinthe corresponding electrode contacting each segment has a second width,and wherein the first width is greater than the second width, with thedifference in the first and second widths being related to a thicknessof an oppositely polarized electrode to be received between the adjacentcommonly polarized electrodes contacting the stacked segments.
 11. Thedevice of claim 10, wherein the at least one of the positive andnegative bus bars is both the positive and negative bus bars, wherein atleast one gas venting space is positioned between the positive andnegative bus bars, and wherein the case substantially contacts theperipheral edges of the bus bars.
 12. The device of claim 7, wherein thepositive and negative electrodes are in the form of a grid comprising apaste, wherein the plates are in contact with an electrolyte, wherein acompressible separator material is positioned between adjacentoppositely polarized electrodes, wherein the compressive force exertedon the electrodes ranges from about 10 to about 100 kPa, wherein thecompression member is separate from the case, and wherein nonconductiveend plates are positioned at either end of the plurality of stackedelectrodes.
 13. The device of claim 11, wherein high and low pressurerelief valves are in fluid communication with the venting space.
 14. Thedevice of claim 1, wherein (c5) is true.
 15. The device of claim 1,wherein (c6) is true.
 16. The device of claim 1, wherein (c7) is true.17. The device of claim 1, wherein (c8) is true.
 18. An electrochemicalenergy storage device, comprising: (a) a plurality of stacked electrodesarranged in a plurality of electrode pairs, each electrode paircomprising an adjacent positive and negative electrode and the pluralityof electrode pairs being arranged such that adjacent electrodes haveopposing polarities; (b) a positive bus bar interconnecting the positiveelectrodes; (c) a case enclosing the stacked electrodes; and (d) anegative bus bar interconnecting the negative electrodes, wherein atleast one of the following is true; (d1) the positive and negative busbars are segmented, each segment physically contacting a correspondingelectrode, wherein the positive bus bar segments are stacked one on topof the other to define a plurality of spaced apart positive electrodesand the negative bus bar segments are stacked one on top of the other todefine a plurality of spaced apart negative electrodes, each of thenegative electrodes being received in a corresponding inter-electrodespace between adjacent positive electrodes and each of the positiveelectrodes being received in a corresponding inter-electrode spacebetween adjacent negative electrodes; (d2) the positive and negative busbars are segmented, each segment physically contacting at least most ofa selected peripheral edge of a corresponding electrode, the adjacentsegments being stacked one on top of the other to form the respectivelypolarized bus bar; (d3) the positive and negative bus bars define atleast one gas venting space positioned between the positive and negativebus bars, wherein the case substantially contacts at least most of theperipheral edges of the bus bars; (d4) the plurality of stackedelectrodes is compressed and maintained in compression by a compressionmember separate from the case; and (d5) the case comprises at least oneoffset member to define a channel between adjacent cases when the casesare positioned side-by-side.
 19. The device of claim 18, wherein (d1) istrue.
 20. The device of claim 18, wherein (d2) is true.
 21. The deviceof claim 18, wherein (d3) is true.
 22. The device of claim 18, wherein(d4) is true.
 23. The device of claim 18, wherein (d5) is true.
 24. Amethod for manufacturing an electrochemical energy storage device,comprising: (a) stacking a plurality of segments of a positive bus bar,each segment being in electrical contact with a positive electrode; (b)stacking a plurality of segments of a negative bus bar, each segmentbeing in electrical contact with a negative electrode; (c) positioningelectrolyte separators between adjacent electrodes; (d) intermeshing thestacked positive electrodes and negative electrodes such that positiveand negative electrodes are positioned in an alternating sequence; and(e) positioning the intermeshed electrodes in a case.
 25. The method ofclaim 24, further comprising: (f) heating the case to expand itsenclosed volume, wherein step (e) occurs while the enclosed volume isthermally expanded.
 26. The method of claim 24, further comprising: (f)compressing the intermeshed positive and negative electrodes before thepositioning step (e).
 27. The method of claim 24, further comprising:(f) welding the adjacent bus bar segments of the positive and negativebus bars to form substantially solid positive and negative bus barsafter the intermeshing step (d).
 28. The method of claim 26, wherein anonconductive end plate is positioned on either side of the compressed,intermeshed positive and negative electrodes and wherein the case isattached to the end plates to form a sealed enclosure for theelectrodes.